Differential signals are becoming popular and important in chip design due to requirements for higher clock and data rates. However, they involve a pair of traces or wires between the driver and the receiver. Typically, one trace carries the positive signal and the other trace carries the negative signal. The negative signal is equal to and opposite the positive signal. As the signals are equal and opposite there is no return signal through ground. The signal that travels down one trace will travel back on the other trace—provided both traces are equal in length and identical.
Unfortunately, in system design, tolerances in printed circuit boards, physical constraints, environmental changes, and so forth, the positive and negative traces are not equal in length, and may not be identical.
FIG. 1 shows the ideal case—when the two traces are of equal length and are identical. Here, there is no time delay. FIG. 2 shows what happens in cases other than the ideal cases. In this case the positive trace is shorter than the negative trace so the signal on the positive trace will arrive earlier than the signal on the negative trace. This gives a time delay that is positive. FIG. 3 shows the reverse—the negative trace is shorter than the positive trace so the negative trace signal arrives earlier than the positive signal trace. This gives a time delay that is negative.
The phase shift difference is a noise source that couples to the ground plane and propagates through the circuitry. Signal integrity is reduced that may cause data error if the phase shift is significant. Furthermore, the phase shift generates a current with a sharp rise time. This may cause EMI problems.